Steam reformers and their systems of product gas collecting conduits are known and their fundamentals are described e.g. in Ullmann's Encyclopedia of Industrial Chemistry, 6th Ed., Vol. 15, p. 332, chapter 2.2.3.
The product gas collecting conduits have the task of taking up up the hot synthesis gas coming from the reaction tubes with a temperature of 900° C. and a high pressure of e.g. 30 bar, and of discharging the same for the further treatment, in particular for cooling by as far as possible recovering the heat contained in the gas. Since the collecting conduits are laid outdoors, they are constructed of a jacket tube and a concentric inner tube with interposed insulation, so as not to dissipate an unnecessary amount of heat to the ambient air. In operation of the reformer, the temperature of the jacket tube on the one hand should be high enough to safely exclude the formation of corrosive condensates in the insulating layer and in particular on the inner wall of the jacket tube, and on the other hand the temperature should not be so high that the steel of the jacket tube is weakened. This is why the insulation disposed between inner tube and jacket tube is designed such that in operation the jacket temperature approximately lies in the range of 150 to 200° C. and an outer insulation of the collecting conduits is omitted completely.
A particular problem consists in setting a temperature as uniform as possible on the entire circumference of the jacket tube, in order to avoid different thermal expansions in the jacket and hence a distortion of the collecting conduit along its length. However, this objective is difficult to achieve, as the upper side of the collecting conduits is exposed to the radiation heat emitted by the reformer bottom and in addition receives an inflow of heat via the ports of the collecting conduit connected with the hot reformer tubes.
The drawing of the European Patent EP 0 799 639 A1 shows a frequently used embodiment of such port. With a view to this drawing the skilled person can understand how heat is transmitted from the hot, gas-carrying inner port tube to the jacket tube of the port and on to the jacket tube of the collecting conduit.
In contrast thereto the bottom sides of the collecting conduits merely are exposed to wind and draft. In general, this leads to the fact that the temperature of the upper side is higher than that of the bottom side, which leads to a stronger thermal expansion of the upper side and hence to bending of the collecting conduit.
A conceivable countermeasure might consist in designing the internal insulation of the collecting conduit with heat transfer coefficients varying over the circumference, as it is proposed in the German patent specification DE 10 2006 022 898 B3. In this way, the amount of heat transmitted from the hot gas flowing in the inner tube through the internal insulation to the jacket tube can be set to vary over the tube circumference such that the above-described external influences are compensated. However, the technical realization of such insulation is expensive.
Therefore, it is the object of the invention to provide a less expensive design of a system of product gas collecting conduits, in which the disadvantages of the prior art at least occur to a reduced extent.